1. Field of the Invention
The invention relates to a power source residual capacity measuring apparatus for detecting a residual capacity of a power source such as primary battery, secondary battery, or the like and also relates to a power source apparatus having a power source residual capacity measuring circuit.
2. Description of the Prior Art
Various types of primary batteries or secondary batteries are installed in a video camera and, mainly, other portable electronic apparatuses.
There is a limitation of a service life of the primary battery installed in such an apparatus. A proper prediction of the service life of the primary battery prevents an erroneous operation of an apparatus using the primary battery, for example, an apparatus such as a video camera or the like and contributes to the effective use of the apparatus. An exchange of the primary battery at a proper timing contributes to the effective use of the primary battery. It is, therefore, desired to properly and accurately predict the service life of the primary battery.
With respect to a secondary battery which can be charged, it is also necessary to properly set the charging timing. In a manner similar to the primary battery, it is desired to properly measure a residual capacity of the secondary battery.
To satisfy such a desire, an apparatus (battery checker) for measuring a residual capacity of a battery such as a dry cell or the like in a non-load state or a predetermined load state has already been known. With reference to the voltage measured by such a battery checker, the user calculates the residual capacity of the primary battery.
An apparatus for integrating a current flowing in a load connected to the battery and for presuming a residual capacity of the battery is also known.
The above voltage measuring apparatus can be realized by a simple circuit construction because it merely detect a battery voltage. Such an apparatus can accurately detect the battery voltage in a predetermined load state or a non-load state.
As shown in FIG. 1, the above voltage measuring method is effective to detect a voltage of a battery in which a change in voltage drop characteristics is typical, for example, a lithium (Li) battery; however, there is a problem such that in case of a battery in which there is hardly a change in voltage drop characteristics until a time point just before the end of the service life, for example, a nickel cadmium (NiCd) battery, there is no voltage change until a time point just before the end of the service life, so that it is difficult to accurately presume the residual capacity from the voltage. In recent years, a battery such as NiCd battery or nickel hydrogen battery in which there is no change in voltage drop characteristics until a time point near the end of the service life of the battery is widely used.
Further, the voltage measuring method causes a phenomenon such that even in case of the same residual capacity, when a load is heavy, the detection voltage is low and, on the contrary, when the load is light, the detection voltage is high. Thus, since the measured voltage fluctuates in dependence on the load state, there is a problem such that the residual capacity of the battery cannot be accurately detected.
On the other hand, according to the current integrating method, since the current flowing in the battery is integrated, the problem in the voltage measuring method mentioned above doesn't occur and the residual capacity of the battery can be calculated. According to the current integrating method, however, the residual capacity of the battery cannot momentarily calculated but must be continuously measured (integrated), so that there is a problem such that a circuit construction to integrate the current becomes complicated. Further, there is also a problem such that when such a circuit is installed in a video camera or the like, its electric power consumption increases.
In the detection of the above residual capacity of the battery mentioned above, it is desired to measure in an apparatus in which the battery is installed, for example, in a state in which the battery is installed in the video camera. In other words, it is desired to remove the battery from the video camera and to measure the residual capacity of the battery in a state in which the battery is connected to the load without using another battery checker. It is requested to reduce the size and costs of the circuit to calculate (measure) the residual capacity of the battery. However, the current integrating method doesn't satisfy such a requirement.
The above problems also similarly occur in another primary battery.
In the secondary battery as well, if the residual capacity can be accurately detected, the battery can be charged at a proper timing. Even in the secondary battery, therefore, it is demanded to accurately detect the residual capacity in a manner similar to the case of the primary battery.
Further, for example, in the case where the secondary battery is detachably installed to the outside of an apparatus like a recent video camera or the like, it is demanded that the user can easily know the residual capacity by the secondary battery itself which was installed to the outside without using the video camera.
Or, in the case where the secondary battery is installed in the video camera or the like, it is demanded that the residual capacity of the battery can be displayed in a view finder or the like of the video camera.